The present invention relates generally to the field of fabricating openings in a substrate and also to apparatuses with these openings. More particularly, the present invention relates to methods for forming openings in a substrate which openings are designed to receive an element which is later placed into the opening and which element includes at least one functional component, and the present invention relates to methods for creating assemblies with the openings.
There are many examples of large arrays of functional components which can provide, produce or detect electromagnetic signals or chemicals or other characteristics. An example of such a large array is that of a display where many pixels or sub-pixels are formed on an array of electronic elements. For example, an active matrix liquid crystal display includes an array of many pixels or sub-pixels which are fabricated on amorphous silicon or polysilicon substrates which are large. As is well known in the art, it is difficult to produce a completely flawless active matrix liquid crystal display (LCD), when the display area is large, such as the LCD""s on modem laptop computers. As the display area gets larger and larger, the yield of good displays decreases. This is due to the manner in which these display devices are fabricated.
Silicon VLSI can be used to produce such an array over a silicon wafer""s surface, but silicon wafers are limited in size, limited in conductivity, and not transparent. Further, processing of large areas on silicon wafers can be expensive. Displays which valve the light coming through them need to be transparent. Single crystal silicon can be bonded to a glass substrate and then etched to remove most of the area to achieve transparency, but this is intrinsically wasteful in that, for the sake of maximizing light transmission, the majority of the processed material is discarded and becomes chemical waste. The under-utilization of the precious die area wastes resources, causes greater amounts of chemical waste to be generated in the process, and is generally inefficient and expensive. Another example is photodiode arrays which may be used to collect solar energy. Large arrays of silicon photodiodes with concentrating lenses have been made by sawing wafers and using a pick and place assembly, but thermal dissipation is poor for large elements, and the small elements require too much assembly time.
Alternative approaches to fabricating arrays such as displays include fabricating the desired circuitry in an amorphous or polycrystalline semiconductor layer which has been deposited on a substrate, such as glass or quartz. These approaches avoid the limitations of the size of the available single crystal silicon wafers, and avoid the cost of the single crystal wafers, but require expensive deposition of the semiconductor layer, and they still require processing of the entire large substrate to form the active elements in an array, still resulting in the production of much chemical waste and wasted resources. These processes also limit the choice of the substrate; for example, plastic substrates cannot be used due to the nature of the processes which deposit the semiconductor layers. Furthermore, amorphous or polycrystalline silicon semiconductor elements do not perform as well as those made from single crystal semiconductor material. For displays, as an example, it is often difficult or impossible to form some of the desired circuitry out of the amorphous or polycrystalline semiconductor materials. Thus, high frequency edge drivers may be impossible to form out of these materials. This results in the difficulty and expense of attaching an electrical lead for each and every row and column of an array, such as an active matrix liquid crystal display array.
As noted above, another difficulty with the existing techniques is that the large number of elements in a large array results in a low probability that all of them will work properly and thus the yield of acceptably good arrays from the manufacturing process is low. Furthermore, there is no possibility of testing any of the elements until the assembly is complete, and then any imperfection in the array must be tolerated or the entire array could be discarded or special and expensive techniques must be used to repair it. These problems result from the fact that the various elements in the array are fabricated on the array rather than separately.
It is possible to separately produce elements, such as pixel drivers and then place them where desired on a different and perhaps larger substrate. Prior techniques can be generally divided into two types: deterministic methods or random methods. Deterministic methods, such as pick and place, use a human or robot arm to pick each element and place it into its corresponding location in a different substrate. Pick and place methods place devices generally one at a time, and are generally not applicable to very small or numerous elements such as those needed for large arrays, such as an active matrix liquid crystal display. Random placement techniques are more effective and result in high yields if the elements to be placed have the right shape. U.S. Pat. No. 5,545,291 describes a method which uses random-placement. In this method, microstructures are assembled onto a different substrate through-fluid transport. This is sometimes referred to as fluidic self assembly (FSA). Using this technique, various blocks, each containing a functional component, may be fabricated on one substrate and then separated from that substrate and assembled onto a separate substrate through the fluidic self assembly process. The process involves combining the blocks with a fluid and dispensing the fluid and blocks over the surface of a receiving substrate which has receptor regions (e.g. openings). The blocks flow in the fluid over the surface and randomly align onto receptor regions.
Thus the process which uses fluidic self assembly typically requires forming openings in a substrate in order to receive the elements or blocks. Methods are known in the prior art for forming such openings and are described in U.S. Pat. No. 545,291. One issue in forming an opening is to create its sidewalls so that blocks will self-align into the opening and drop into the opening. The substrate having openings in the glass layer 10 may be used as a receiving substrate to receive a plurality of elements by using a fluidic self assembly method. FIG. 1A shows an example where a separately fabricated element 16 has properly assembled into the opening 14. However, it has been discovered that at times, an element 16 will not properly assemble into an opening 14 due to the fact that the element 16 becomes turned upside down and then lodges in the top of the opening 14. An example of this situation is shown in FIG. 1B. Often times, the inverted element 16 lodges into the opening 14 so tightly that it remains in the opening and prevents non-inverted elements from falling into the opening 14. Thus, the opening at the end of the assembly process will typically not be filled with an element or perhaps worse, may still contain an inverted element lodged at the top of the opening 14.
FIGS. 2A through 2D show an example in the prior art for creating a plurality of openings in a receiving substrate which is designed to receive a plurality of separately fabricated elements which are deposited into the openings through fluidic self assembly. The method shown in FIGS. 2A through 2D begins by, in one example, thermally growing a silicon dioxide layer on a silicon substrate 20. The resulting structure is shown in FIG. 2A with the silicon dioxide layer disposed over the silicon substrate 20. Then, a photoresist material may be applied, and exposed through a lithographic mask and then developed to produce a patterned mask formed from the developed photoresist. Then an etching solution is applied-to etch through the patterned mask to create an opening 24 in the silicon dioxide layer .22. The resulting structure is shown in FIG. 2B. Then, the silicon dioxide layer 22 with its opening 24 is then used as a patterned mask to etch the silicon layer 22 to create the opening 26 in the silicon layer 20 as shown in FIG. 2C. This etching of the silicon layer 20 may be performed with a KOH etchant or with an EDP etchant as described in U.S. Pat. No. 5,545,291. After etching the opening 26 in the silicon layer 20, the silicon dioxide layer 22 is removed, for example, by an etch in a hydrofluoric acid solution. This results in the structure shown in FIG. 2D where the opening 26 is now ready to receive a separately fabricated element through an assembly process, such as for example, fluidic self assembly or perhaps a pick and place procedure. The structure shown in FIG. 2D has the drawback that a monocrystalline silicon layer is required in order to use the KOH etch to form the hole.
Often, it will be desirable to obtain a deep enough opening without making the opening too wide. This, of course, will depend on the shape, which is typically predetermined, of the separately fabricated element or block which is to be deposited into the opening. Naturally, the shape of the opening is designed to fit substantially the shape of the separately fabricated element. Often times, it is necessary to obtain an angle in the opening which is steeper than a 45xc2x0 angle. These various requirements and the problems associated with inverted elements which become lodged in openings have resulted in attempts to improve the methods for fabricating the openings in a receiving substrate.
From the above, it is seen that it is desirable to provide methods for forming openings in a receiving substrate and to provide methods for creating assemblies with these openings.
The present invention provides various methods for creating an opening in a substrate and also provides apparatuses resulting from these methods. In one example of a method according to the present invention, an opening which has a predetermined cross-sectional shape is created in a substrate. The opening is designed to receive an element which is separately fabricated and which typically includes at least one functional component and which is placed into the opening in a process such as pick and place or fluidic self assembly. In this example, the method involves etching the substrate through a first patterned mask for a first portion of an etch time and etching the substrate through a second patterned mask for a second portion of the etch time. In one particular example of this method, the first and second patterned masks are different.
In another example of a method according to the present invention, an opening which has a predetermined cross-sectional shape in a substrate and which is designed to receive an element which is placed into the opening is created by applying a patterned mask over a material which is sensitive to electromagnetic radiation and exposing the material and the patterned mask to electromagnetic radiation which is project obliquely to a surface of the material such that some of the electromagnetic radiation impinges into a first portion of the material which is under the patterned mask. The patterned mask is removed and a second portion of the material which was under the patterned mask is also removed.
According to another aspect of the present invention, a method is provided for forming an opening in a first layer which includes silicon dioxide. In this method, a second layer is deposited over the first layer which includes silicon dioxide, and a metal adhesion layer, such as a tungsten layer is deposited over the-second layer. The metal adhesion layer is patterned and the second is patterned to expose a portion of the first layer which is then etched.
The present invention also provides a substrate having at least one opening which is designed to receive an element having a predetermined shape. The element is fabricated separately and assembled into the opening. The opening includes in a region near its top edge a beveled surface which in one exemplary embodiment is designed to decrease the frequency of inverted elements from being wedged into the top of the opening.
According to another aspect of the invention, a method for creating an opening in a layer is described. The opening is for receiving an element which is placed into the opening. The method includes forming a first layer on a substrate, depositing a second layer over the first layer, and ablating selectively the second layer at at least one desired region to create an opening in the second layer at the at least one desired region, wherein the ablating stops automatically at the first layer.
According to another aspect of the invention, another method for creating an opening in a substrate is described. The opening is for receiving an element which is fabricated on another substrate and is placed in the opening. The method includes forming an organic layer on a glass substrate and forming an opening in the organic layer.
According to another aspect of the invention, a method for etching glass in an etching solution is described. The etching solution has certain described concentrations of hydrofluoric acid, a counter acid (e.g. HCl, HBr, HI, HNO3, or H2SO4) and water and the etching of the glass with the etching solution is performed at a reduced temperature in the range of about 0xc2x0 C. to about 10xc2x0 C.
These aspects as well as other features of the present invention will be described further below.